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The chmod command (abbreviated from change mode) is a shell command and C language function in Unix and Unix-like environments. The command is based on the function. When executed, it can change file system modes of files and directories. The modes include permissions and special modes.


[edit] History

A chmod command first appeared in AT&T Unix version 1, and is still used today on Unix-like machines.

[edit] Usage

The chmod command options are specified like this:

$ chmod [options] mode[,mode] file1 [[file2 ...]]

To view what the permissions currently are, type:

$ ls -l file

The function prototype is defined:

int chmod(const char *path, mode_t mode);

[edit] Command line options

The chmod command has a number of command line options that affect its behavior. The most common options are:

  • -R: Changes the modes of directories and files recursively
  • -v: Verbose mode; lists all files as they are being processed

[edit] String modes

See also: Symbolic notation of file system permissions

To the chmod utility, all permissions and special modes are represented by its mode parameter. One way to adjust the mode of files or directories is to specify a symbolic mode. The symbolic mode is composed of three components, which are combined to form a single string of text:

$ chmod [references][operator][modes] file1 ...

The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to “all”. They are represented by one or more of the following letters:

Reference Class Description
u user the owner of the file
g group users who are members of the file's group
o others users who are not the owner of the file or members of the group
a all all three of the above, is the same as ugo

The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:

Operator Description
+ adds the specified modes to the specified classes
- removes the specified modes from the specified classes
= the modes specified are to be made the exact modes for the specified classes

The modes indicate which permissions are to be granted or taken away from the specified classes. There are three basic modes which correspond to the basic permissions:

Mode Name Description
r read read a file or list a directory's contents
w write write to a file or directory
x execute execute a file or recurse a directory tree
X special execute which is not a permission in itself but rather can be used instead of x. It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least 1 execute permission bit already set (either user, group or other). It is only really useful when used with '+' and usually in combination with the -R option for giving group or other access to a big directory tree without setting execute permission on normal files (such as text files), which would normally happen if you just used "chmod -R a+rx .", whereas with 'X' you can do "chmod -R a+rX ." instead
s setuid/gid details in Special modes section
t sticky details in Special modes section

The combination of these three components produces a string that is understood by the chmod command. Multiple changes can be specified by separating multiple symbolic modes with commas.

[edit] String mode examples

For example, the following command would be used to add the read and write permissions to the user and group classes of a file or directory named sample:

$ chmod ug+rw sample
$ ls -ld sample
drw-rw----   2 unixguy  unixguy       96 Dec  8 12:53 sample

This command removes all permissions, allowing no one to read, write, or execute the file named sample.

$ chmod a-rwx sample
$ ls -l sample
----------   2 unixguy  unixguy       96 Dec  8 12:53 sample

The following command changes the permissions for the user and the group to read and execute only (no write permission) on sample.

Sample file permissions before command
$ ls -ld sample
drw-rw----   2 unixguy  unixguy       96 Dec  8 12:53 sample
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x---   2 unixguy  unixguy       96 Dec  8 12:53 sample

[edit] Octal numbers

See also: Octal notation of file system permissions

The chmod command also accepts three and four-digit octal numbers representing modes. See the article mentioned above for more information. Using a four-digit octal number to set the modes of a file or directory named sample would look something like this:

$ chmod 0664 sample

Assuming that the setuid, setgid and sticky bits are not set, this is equivalent to:

$ chmod 664 sample


$ chmod +r,-x,ug+w sample

[edit] Special modes

See also: Additional file system permissions

The chmod command is also capable of changing the additional permissions or special modes of a file or directory. The symbolic modes use s to represent the setuid and setgid modes, and t to represent the sticky mode. The modes are only applied to the appropriate classes, regardless of whether or not other classes are specified.

Most operating systems support the specification of special modes using octal modes, but some do not. On these systems, only the symbolic modes can be used..

[edit] Command line examples

command explanation
chmod +r file read is added for all
chmod -x file execute permission is removed for all
chmod u=rw,go= file read and write is set for the owner, all permissions are cleared for the group and others
chmod +rw file change the permissions of the file file to read and write for all.
chmod -R u+w,go-w docs/ change the permissions of the directory docs and all its contents to add write access for the user, and deny write access for everybody else.
chmod 0 file removes all privileges for all
chmod 666 file sets read and write access for the owner, the group, and all others.
chmod 0755 file equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1). The 0 specifies no special modes.
chmod 4755 file the 4 specifies set user ID and the rest is equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1).
find path/ -type d -exec chmod a-x {} \; removes execute permission for all directories (cannot list files) in tree starting from path/ (use '-type f' to match files only).
find path/ -type d -exec chmod a+x {} \; allows directory browsing (ls for example) for all users if you've reset permissions for Samba write access.
chmod -R u+rwX,g-rwx,o-rwx directory set a directory tree to rwx for owner directories, rw for owner files, --- for group and others.
chmod -R a-x+X directory remove the execute permission on all files in a directory tree, while allowing for directory browsing.

[edit] Function details

The function takes a parameter of type mode_t, which is a bitfield composed of various flags:

flag octal value purpose
S_ISUID 04000 set user ID on execution
S_ISGID 02000 set group ID on execution
S_ISVTX 01000 sticky bit
S_IRUSR, S_IREAD 00400 read by owner
S_IWUSR, S_IWRITE 00200 write by owner
S_IXUSR, S_IEXEC 00100 execute/search by owner
S_IRGRP 00040 read by group
S_IWGRP 00020 write by group
S_IXGRP 00010 execute/search by group
S_IROTH 00004 read by others
S_IWOTH 00002 write by others
S_IXOTH 00001 execute/search by others

Where alternate flag names are given, one of the pair of names might not be supported on some OSs. The octal values of the flags are summed or combined in a bitwise or operation to give the desired permission mode.

Various error codes can be returned, and these are detailed in the associated man page for the function.

[edit] See also

[edit] External links

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